According to mass spectrometry, the molecules of a sample component are ionized. Then, the ions are electromagnetically fractionated by mass (mass number), and the ion intensity is measured. The former part for ionization is called an ionization unit (ionizer), and the latter part for mass fractionation is called a mass spectrometry unit (mass spectrometer). The mass spectrometry is typical of instrumental analysis methods because of its high sensitivity and precision, and is applied to a wide range of fields including material development, product inspection, environmental research, and biotechnology. Most mass spectrometers are used in combination with a component separator such as a gas chromatograph (GC). In this case, however, the following problems arise. A sample needs to be refined for component separation. As long as several tens of minutes is required till the end of component separation. A sample component may change in quality or be lost during component separation. Component separation requires a deep knowledge and considerable experience.
For a quick, simple, and high-precision measurement, a “direct measurement method” is also employed, in which a mass spectrometer singly performs measurement without being combined with a component separator.
Ionizers used in the “direct measurement method” are greatly different in principle and structure. An ion attachment mass spectrometer is advantageous because it can analyze the mass of a gas to be detected without dissociation. Conventional ion attachment mass spectrometers are reported in non-patent references 1 to 3 and patent reference 1.
FIG. 5 shows a conventional ion attachment mass spectrometer that evaporate a solid or liquid sample and measures the mass number of the sample.
In FIG. 5, an ionization chamber 100 and sample evaporation chamber 110 are arranged in a first cell 130. A mass spectrometer 160 is arranged in a second cell 140. A vacuum pump 150 evacuates the first cell 130 and second cell 140. Hence, all the ionization chamber 100, sample evaporation chamber 110, and mass spectrometer 160 are maintained in a pressure atmosphere (vacuum) lower than the atmospheric pressure.
An emitter 107 made of alumina silicate containing an alkali metal oxide as of lithium is heated to generate and emit positively charged metal ions 108 such as Li+. The sample evaporation chamber 110 is arranged separately from the ionization chamber 100, which chambers are connected by a connecting pipe 120.
A probe 111 is inserted into the sample evaporation chamber 110 from the outside (from the left in FIG. 5) to heat a sample cup 112 provided at the distal end of the probe 111. Since the sample cup 112 is filled with a sample 113, the sample 113 is evaporated and releases neutral gas phase molecules 106 of the sample 113 as a gas to be detected in the sample evaporation chamber 110. The neutral gas phase molecules 106 move toward the ionization chamber 100 by self-diffusion and enter it.
In the ionization chamber 100, the neutral gas phase molecules 106 are ionized, generating ions.
Finally, the generated ions are transported from the ionization chamber 100 to the mass spectrometer 160 upon receiving a force from an electric field. The mass spectrometer 160 fractionates the ions for respective masses and detects them.
The metal ions 108 attach to portions of the neutral gas phase molecules 106 that have charge bias. The molecules (ion-attached molecules 109) with the metal ions 108 attached form ions that are positively charged overall. The neutral gas phase molecules 106 do not decompose because attaching energy (energy for attachment which turns into excess energy after attachment) is very small. The ion-attached molecules 109 therefore act as molecular ions without changing from the original molecular form.
Molecular ions that keep the original molecular form, like the ion-attached molecules 109, will be called fragment-free ions. When the ion-attached molecules 109 generate ions to be detected, these ions will be called fragment-free ions to be detected.
If, however, the ion-attached molecules 109 are left stand (keep holding excess energy) after the metal ions 108 attach to the neutral gas phase molecules 106, the excess energy breaks bonds between the metal ions 108 and the neutral gas phase molecules 106. The metal ions 108 move apart from the neutral gas phase molecules 106 and return to original neutral gas phase molecules 106. To prevent this, the ion-attached molecules 109 are caused to frequently collide against gas molecules by introducing gas such as N2 gas (nitrogen gas) from a gas cylinder 170 into the ionization chamber 100 up to a pressure of about 50 to 100 Pa (flow rate of 5 to 10 sccm). Then, excess energy held by the ion-attached molecules 109 moves to the gas molecules to stabilize the ion-attached molecules 109.
This gas has an important function in the ion attachment process to make metal ions 108 emitted by the emitter 107 collide against each other so that the metal ions 108 are decelerated and easily attach to the neutral gas phase molecules 106. This gas is called a third-body gas.
As shown in FIG. 5, the third-body gas cylinder 170 is connected to the ionization chamber 100 via a pipe so that it can introduce a third-body gas into the ionization chamber 100.
In the above-mentioned ion attachment mass spectrometer, the emitter is arranged on the central axis and emits the metal ions 108 along the central axis (lateral direction in FIG. 5). This structure requires the sample evaporation chamber 110 in addition to the ionization chamber 100. The opening of the sample cup 112 arranged inside the sample evaporation chamber 110 is perpendicular (upward in FIG. 5) to the central axis. The neutral gas phase molecules 106 are released in a direction (upward in FIG. 5) perpendicular to the central axis.
One reason of this arrangement is as follows. The metal ions 108, which are primary particles used for ionization, are low-speed ions and are effectively affected by an electric field within the ionization chamber 100, similar to the generated ion-attached molecules 109. The metal ions 108 need to be emitted along the central axis, and thus the emitter is located on the central axis of the structure.
Prior Art References
Patent Reference
Patent Reference 1: Japanese Patent Laid-Open No. 6-11485
Non-Patent References
Non-Patent Reference 1: Hodge (Analytical Chemistry vol. 48, No. 6, p. 825 (1976))
Non-Patent Reference 2: Bombick (Analytical Chemistry vol. 56, No. 3, p. 396 (1984))
Non-Patent Reference 3: Fujii (Analytical Chemistry vol. 61, No. 9, p. 1026, Chemical Physics Letters vol. 191, no. 1.2, p. 162 (1992))